362 3. A-ETHYLMALEIMIDE 



(see accompanying tabulation) (Tsen and Collier, 1960) and yet hemolyzes 

 only at high concentrations. In this respect, iV-ethylmaleimide is like 

 iodoacetate. There is a rapid and extensive reaction of A^-ethylmaleimide 



with erythrocytic SH groups as shown by a decrease in absorption at 300 

 m//, the reaction reaching completion within 60 min at pH 6.8, 25^, and 

 2 mM xV-ethylmaleimide (Morell et al, 1959). About 1400 //moles of N- 

 ethylmaleimide react per 100 ml of erythrocytes, this being equivalent to 

 approximately 3 moles of SH per mole of hemoglobin. There are about 

 2 //moles of glutathione per millimeter of erythrocytes and A'-ethylmaleimide 

 reacts with these completely within 5 min, whereas p-MB cannot penetrate 

 readily into the cells (Jacob and Jandl, 1962). Glucose utilization and 

 lactate formation are readily inhibited by iV-ethylmaleimide but not by 

 p-MB. The facilitated transport of glucose is progressively depressed by 

 iV-ethylmaleimide at concentrations from 3.9 mM to 12.1 mM, but in 

 contrast to the reaction with glutathione the rate of inactivation of the 

 glucose carrier is slow, maximal inhibitions being reached only after sev- 

 eral hours (Dawson and Widdas, 1963). The kinetics can be formally 

 represented if there are assumed to be three types of SH group or carrier 

 — 70% of a faster reacting component, 25% of a slower reacting component, 

 and 5% of an unreactive component. The reaction has the fairly high 

 activation energy of 27 kcal/mole {Q^q around 4) and the rate increases 

 with rise in the pH from 5.5 to 8.5. Increase of the glucose concentration 

 has no effect on the inhibition until it is greater than 4 mM and then the 

 rate of reaction with the carrier is increased. That this is a selective effect 

 on a glucose carrier is indicated by the complete lack of effect on malon- 

 amide penetration. 



iV-Ethylmaleimide causes a loss of K+ from duck erythrocytes but, in 

 contrast to other inhibitors, also causes some loss of Na+, this being ac- 

 companied by marked suppression of glucose uptake and glycolysis (Toste- 

 son and Johnson, 1957). There is also loss of water from the cells. K+ 

 loss from human erythrocytes also occurs when exposed to A'-ethylmalei- 



